Toll-like receptors (TLRs) recognize the pathogen-associated molecular patterns (PAMPs) and play a pivotal role in modulating the immune response against invading pathogens. To date, many teleost TLRs have been identified by genome and transcriptome analysis. The orthologs involved in the TLR signalings have also been identified in teleosts. The comparison of the TLR functions in teleosts and mammals may provide significant information that will aid our understanding of host-pathogen interactions in teleosts. In this paper, we summarize of the literature regarding teleost TLRs, focusing on the similarities and differences between TLR mechanisms in teleosts and mammals. In addition, perspectives for future studies on teleost TLRs and the application of TLRs to aquaculture are discussed.

Edwardsiella tarda causes a serious economic loss in olive flounder Paralichthys olivaceus farming industry. Extracellular products (ECP) of the pathogen contain various bio-molecules, some of which are responsible for the virulence. ECP of E. tarda virulent strain, KE-1, was used to investigate the effects on the nature of phagocytes isolated from olive flounder. High doses of ECP down-regulated phagocytosis and bactericidal activity on engulfed bacteria in phagocytes and chemiluminescence (CL) activity of macrophages. However, production of nitric oxide of macrophages soared under high concentrations of ECP. In contrast, low doses of ECP induced increases in bacterial invasion activity and CL activity. These results suggest that appropriate amounts of ECP could effectively stimulate cellular immunity, and that E. tarda ECP is a good candidate for vaccine adjuvant.

A visceral mycosis occurred in ayu Plecoglossus altivelis larvae at Yamanashi Prefectural Fisheries Technology Center, Japan, in 2007 and 2008. Cumulative mortalities due to the disease were 19-33%. Most diseased fish were characterized by the opaque abdomen. Abundant non-septate hyphae with a width of approximately 5 μm were observed in the opaque areas. Fungi isolated from diseased fish were all identified as Pythium flevoense based on the morphological characteristics and sequence analysis of the 5.8S rDNA and adjacent ITS regions. Histopathological examinations showed that non-septate hyphae were present in the airbladder, kidney, intestine, pancreas, spleen, abdominal cavity, musculature and spinal cord. Heavy hyphal propagation in the airbladder and rhexis of the organ suggested that accidental ingestion of P. flevoense into the airbladder was the prime cause of this disease.

The mode of intra-ovum infection by Flavobacterium psychrophilum, which causes bacterial cold-water disease and rainbow trout fry syndrome, was investigated in eggs of rainbow trout Oncorhynchus mykiss. When newly spawned unfertilized eggs were immersion-challenged with F. psychrophilum at concentrations of 107 CFU/mL or higher and subsequently inseminated and water-hardened, the bacteria were detected in the egg contents. The infection rate in eggs was related to the bacterial concentration in a dose-dependent manner. When mature female rainbow trout were intraperitoneally injected with F. psychrophilum (3.3 × 109 CFU/fish) 5-9 days prior to ovulation, intra-ovum infection was found in eggs spawned from the fish showing heavy contamination of the bacterium in the ovarian fluid (> 106 CFU/mL). These results indicate that F. psychrophilum can contaminate the surface of eggs in ovarian fluid and enter eggs during water-hardening. Live bacterial counts within experimentally infected eggs and immuno-staining observations on the eggs suggested that F. psychrophilum passively entered into the eggs (< 10 CFU/egg) and subsequently grew to over 107 CFU/egg in the perivitelline space by the eyed egg stage. Multiplication of the bacterium in the perivitelline space did not affect the survival of eggs.

In experimental infection with Streptococcus parauberis in Japanese flounder Paralichthys olivaceus, we investigated suitable routes of inoculation for a reproduction of natural infection: intraperitoneal injection, intravascular one in tail, intramuscular one into a lateral muscle or erecting and depressing muscles of dorsal fin, and subcutaneous one at a base of dorsal fin or outside preopercular bone. Among these routes, the subcutaneous injection resulted in the challenge-dose-dependent mortality. Disease and histopathological signs of the dead fish were similar to those of spontaneously affected fish. The subcutaneous injection at a base of dorsal fin or outside preopercular bone is a recommended method at a challenge of S. parauberis.

The serological relationships among 13 strains of Edwardsiella ictaluri isolated from wild ayu Plecoglossus altivelis in Japan were studied, using other E. ictaluri strains isolated from catfish as references. Slide agglutination with rabbit antisera revealed that all E. ictaluri strains from ayu were antigenically homogeneous and closely related to two Vietnamese strains and three Indonesian strains from striped catfish Pangasius hypophthalmus. The type strain of E. ictaluri (JCM1680 = ATCC33202) isolated from channel catfish Ictalurus punctatus and one Indonesian strain from striped catfish belonged to a different serogroup. These results were corroborated by serum absorption tests with the homologous and heterologous antigens.